1. Field of the Invention
The present invention generally relates to an optical semiconductor device and a method for manufacturing the optical semiconductor device. More particularly, the present invention relates to an optical semiconductor device having a diffusion region that includes a light emitting facet and has impurities diffused therein, and a method for manufacturing the optical semiconductor device.
2. Description of the Related Art
In recent years, optical semiconductor devices such as laser diodes (LDs) that emit light are used in the fields of optical communications and optical storage media. In high-power laser diodes, optical damage called COD (Catastrophic Optical Damage) has become a problem and caused decreases in reliability. COD is caused when the light emitting facet through which laser light is emitted serves as an absorbing area for the laser light.
To prevent COD, there has been a technique by which the bandgap of the activation layer of the light emitting facet for emitting laser light is made wider than the activation layer located on the inner side of the light emitting facet. By this technique, absorption of the laser light on the light emitting facet is prevented, and COD can be prevented accordingly. To widen the bandgap of the activation layer of the light emitting facet, a diffusion region having impurities diffused therein is often formed.
However, if the impurity concentration in the diffusion region is too high, the light absorption by the free carriers becomes large, and the problem of COD arises again. Japanese Patent Application Publication Nos. 5-218593 and 2006-303211 disclose techniques by which the diffusion region having impurities diffused therein is subjected to another heat treatment.
In optical semiconductor devices, breaking on the light emitting facet caused by COD is often found through an APC (Auto Power Control) reliability test and pulse I-L measurement carried out to evaluate optical output power characteristics. As a result, the problem of poorer reliability arises.
With a technique of subjecting the diffusion region to another heat treatment as disclosed in Japanese Patent Application Publication Nos. 5-218593 and 2006-303211, there is the problem of defects formed in the surface of a compound semiconductor layer such as a cladding layer or an activation layer formed on the substrate.